Harvesting rain water

This small 500 litre tank holds water for pot plants on the verandah and vegetables in the front yard. We bought it for $49 on sale at Bunnings.It hasn't rained here for about six weeks. Our large green water tank on the front verandah is empty but I am hoping for rain to start falling soon. I checked the radar, rain is on the way. Hopefully, in about 30 minutes, even before I post this, it